NONLINEAR EFFECTS IN DIPOLE SOLVATION .2. OPTICAL SPECTRA AND ELECTRON TRANSFER ACTIVATION

摘要

We present a theoretical analysis of the effect of nonlinear dipole solvation on steady-state optical spectra and intramolecular electron transfer (ET) reactions. The solvation nonlinearity is attributed to saturation of a dipolar liquid produced by the solute dipole. The treatment explores the perturbation expansion over the solute-solvent dipolar interaction truncated in the form of a Pade approximant. The optical line shape and the free energies along the ET reaction coordinate are related to the chemical potential of solvation of a fictitious solute with a complex-valued dipole moment. Due to solvent dipolar saturation the spectrum of dipolar fluctuations is confined by a band of the width 2E(lim). Solvation nonlinearity was found to manifest itself for optical transitions with high dipole moments in the initial state, most often encountered for emission lines. In this case, the spectral line approaches the saturation boundary E-lim bringing about ''line squeezing'' and decrease of the line shift compared to the linear response prediction. In the nonlinear region, the line shift dependence on the solute dipole variation Delta m switches from the quadratic linear response form proportional to Delta m(2) to a linear trend proportional to Delta(m). The bandwidth may pass through a maximum as a function of Delta m in the saturation region. Nonlinear solvation results thus in a narrowing of spectral lines. For a transition with Solute dipole enhancement, the bandwidth in emission Delta(e) is therefore lower that in absorption Delta(a):Delta(e)